Effects of progesterone and synthetic luteinizing hormone releasing hormone on the release of luteinizing hormone during sexual maturation in the hen (Gallus domesticus).

Single intramuscular injections of 0-5 mg progesterone/kg resulted in increased LH secretion in laying hens but not in pullets with completely undeveloped sexual organs. Injections of the steroid were first able to stimulate LH release 8-10 weeks before the onset of lay when the comb, ovary and oviduct had started to grow and basal plasma LH concentrations were beginning to rise. At this time, injecitons of 10 mug synthetic LH-RH/kg resulted in an incremental change in plasma LH levels of around 26 ng/ml. A similar incremental change was observed after giving the same dose of LH-RH to pullets with no signs of sexual development. Three to four weeks before the first eggs were laid, basal plasma LH levles started to fall, the pituitary became progressively more insensitive to synthetic LH-RH and injections of 0-5 mg progesterone/kg resulted in a reduced LH response. Ten mug LH-RH/kg caused incremental changes in blood levels of LH of less than 5 ng/ml. The final stage of sexual maturation occurred during the week before the onset of lay and was characterized by a rapid growth of large yolky ovarian follicles and a further fall in the sensitivity of the pituitary to synthetic LH-RH. However, injections of 0-5 mg progesterone/kg resulted in a prolonged release of LH. These observations are discussed in relation to the maturatio of the positive feedback mechanism by which progesterone stimulates the secretion of LH.